This invention relates to semiconductor processing equipment, and more particularly to apparatus for performing high-temperature processes upon semiconductor wafers in a tube furnace.
Thermal processing equipment for processing semiconductor wafers is described in U.S. Pat. Nos. 4,950,156, 4,992,044 and 5,064,367, issued to Ara Philipossian and assigned to Digital Equipment Corporation. These patents describe a tubular cantilever construction where semiconductor wafers are protected from exposure to ambient air and particulates. Oxygen, water vapor and airborne particulates found in ambient air can react with the wafers and adversely affect their chemical and physical properties. By means of a tubular cantilever into which the loaded wafer boats are inserted, the wafers are protected against particulates, and, by flowing an inert gas through the tube, also protected from moisture and air. The tubular cantilever also reduces the amount of contaminants seen by the wafers inside the furnace tube itself since the wafer is isolated by the tubular cantilever from the walls of the furnace tube. Various furnace design improvements as described in these patents assure a uniform flow of the reaction gases through the furnace tube to prevent contaminants from entering the reaction tube and adversely affecting the wafers, and to ensure efficient removal of the gases fed to the furnace.
In U.S. Pat. No. 4,950,156, an inert gas curtain is described for reducing the amount of ambient air allowed to enter the furnace during the time the tubular cantilever is removed. U.S. Pat. No. 4,992,044 discloses an exhaust system for removing spent gases from one end of the tubular cantilever in a symmetrical manner. U.S. Pat. No. 5,064,367 describes a conical gas inlet for a tubular processing system to reduce gas recirculation and eddys.
In the thermal processing systems disclosed in each of the U.S. Pat. Nos. 4,950,156, 4,992,044 and 5,064,367, a tubular cantilever containing the wafers to be processed is inserted concentrically within a furnace tube, so that an annular space exists between the outer wall of the tubular cantilever and the inner wall of the furnace tube. The spacing must be sufficient to allow free movement of the tubular cantilever in and out of the furnace tube without mechanical interference. During the wafer processing operation, this annular space becomes part of the dead space inside the furnace that must be filled with inert or reactant gases, but the space does not contribute any useful processing function since it is too far removed from the wafers themselves.
A particular construction of a tubular cantilever for use in thermal processing equipment for semiconductor wafers is disclosed in U.S. Pat. No. 4,543,059, issued to Whang and Wollmann. In the cantilever reactor of U.S. Pat. No. 4,543,059, an inherent part of the geometry is that an outer annular region is created within the reactor (between the inner wall of the furnace tube and the outer wall of the cantilever). Depending on the maximum allowable wafer diameter, and the baffle configuration, this region can account for a significant fraction of the total volume of the reactor. Furthermore, the patent shows that the annular region should be equipped with its own exhaust so as to cause minimal disruption of gas flow within the reactor and to ensure efficient gas purging. In an attempt to address this issue, the patentees disclose a bypass tube having a cross sectional area which is roughly ten percent of the main exhaust port. The patentees assumed that roughly ten percent of the total flow rate would enter the outer annular region and be effectively exhausted through the bypass tube. Subsequently, however, residual gas analysis in conjunction with the residence time distribution technique, conducted by the assignee herein, shows that the bypass design of U.S. Pat. No. 4,543,059 is essentially ineffective in ensuring satisfactory fluid dynamics within the outer annular region. Two independent approaches verify this.
The first approach uses the residence time distribution technique to determine the mean residence time of gas molecules in the outer annulus of a reactor containing a varying number of baffles in its main chamber. Theoretically, for an open system (i.e. a system which has an inlet and an outlet) placing more baffles in the main reactor chamber should increase the flow resistance in the main chamber, thus forcing more gas through the outer annulus. Therefore, a five-baffle system is expected to have a greater outer annular flow rate compared to three- and one-baffle systems, respectively. Since mean residence time and flow rate are inversely proportional, the addition of more baffles in the main chamber should result in smaller mean residence times in the outer annular region (again, assuming that the outer annular region is independently exhausted). But the results of analysis indicate this not the case. For a given flow rate (flow rate at several differing levels) only very slight differences in the mean residence time as a function of baffle configuration were noted, these differences being well within the margin of experimental error. The results imply that the outer annular region lacks its independent exhaust (i.e. the bypass is not operating efficiently). A great majority of the gases entering the outer annulus, exit that region after some time, by backing off through the outer annulus and re-entering the main chamber. The gases finally leave the reaction system via the main exhaust port.
The second, more direct, approach involved comparing the mean residence time of gases in the outer annulus of a conventional cantilever-equipped, furnace (i.e. bypass tube according to U.S. Pat. No. 4,543,059) with a reactor having a completely plugged bypass tube. The results indicate that there are no marked difference in the outer annular mean residence times between the two configurations. This again shows that the bypass tube is not effective in offering independent exhaust capability to the outer annular region.
Accordingly, one of the objectives of the reactor to be described herein is to avoid the problems inherent in the reactor design of the U.S. Pat. No. 4,543,059 in regard to recirculation of gas from an annular space between inner walls of the furnace and outer walls of the cantilever tube. Another objective is concerned with improving plug flow characteristics.
Continuous flow reactors are of two kinds, plug flow reactors, and continuously stirred tank reactors (CSTR). Plug flow reactors assume complete mixing in the radial direction, but allow for no diffusion/dispersion in the longitudinal (axial) direction. As a result, the compositional profiles are uniform over any cross-sectional area perpendicular to the flow. The CSTR is the opposite extreme of the plug flow reactor. The essential feature of a CSTR is the assumption of complete uniformity of concentration throughout the reactor. In CSTRs, the concentration in the effluent stream is equivalent to the concentration everywhere within the vessel. In order to approach this ideal mixing pattern, it is necessary that the feed be intimately mixed with the contents of the reactor in a time interval that is very small compared to the mean residence time of fluids flowing through the vessel. Given the highly ideal nature of both types of reactors, it should be noted that both systems can achieve perfect growth rate uniformity in a silicon oxidation furnace. The furnaces herein described are necessarily operated in the plug flow mode, so the CSTR will not be treated further.
It can be demonstrated that a high degree of SiO.sub.2 growth rate uniformity can be achieved in a plug flow reactor. Even though the first and last wafers in the reactor experience the approaching oxygen boundary (the "plug" of oxygen flowing through the system) at different points in time, both wafers undergo identical compositional exposures to oxygen for identical amounts of time. A model of a thermal processing furnace, using flow visualization studies, illustrates that conventional furnaces (whether equipped with cantilever loading or not) exhibit conditions which are far from those of ideal CSTR or plug flow reactor conditions. Also, it is seen that attaining ideal CSTR conditions is near impossible due to the large length-to-width ratio associated with thermal furnaces.
An inherent problem associated with most prior thermal processing furnaces is the fact that the reactant gas jet which enters the reaction vessel through the injector nozzle has a characteristic elongated shape which extends approximately one-third of the way through the reactor. This undesirable feature induces unwanted turbulence within the system and causes premature and incomplete local mixing of reactant gases in the longitudinal direction. Any effort to prevent this longitudinal thrust should result in a flow pattern which approaches plug flow conditions and should help ensure that the entire wafer load is exposed to uniform compositional conditions within the reactor.